Eclipse is a cigarette that primarily heats rather than burns tobacco. R.J. Reynolds Tobacco Company (RJRT) has previously reported the results of in vitro toxicity studies comparing Eclipse with University of Kentucky 1R5F and 1R4F reference cigarettes. To characterize the differences between Eclipse and very low yielding/ultra low-"tar" (vULT) tobacco-burning cigarettes, RJRT conducted a comparative evaluation of the genotoxicity and cytotoxicity of mainstream cigarette smoke condensate (CSC) from Eclipse and three vULT tobacco-burning cigarettes (Now 83 Box, Merit Ultima and Carlton Soft Pack) as well as the leading ultra low-"tar" (ULT) brandstyle (Marlboro Ultra Lights) under four smoking regimens: (1) FTC-35 ml puff volume every 60 s for a 2 s duration (35/60/2); (2) 50/30/2, 0% vents blocked; (3) Massachusetts-45/30/2, 50% vents blocked; (4) Canadian-55/30/2, 100% vents blocked. Ames testing indicated that Eclipse CSC was less (P<0.05) mutagenic than CSC from the four cigarettes under all smoking regimens when compared on a revertants per mg Total Particulate Matter (TPM) basis. When mutagenicity was calculated on a revertants per cigarette basis the mutagenicity of Eclipse CSC was lower (P<0.05) than the mutagenicity of Merit Ultima, Carlton Soft Pack, and Marlboro Ultra Lights, regardless of the puffing regimen. On a per cigarette basis, the calculated mutagenicity of Eclipse was higher (P<0.05) than Now 83 Box cigarettes in the FTC and 50/30/2 regimens but lower (P<0.05) in the Massachusetts and Canadian regimens. Eclipse CSC was less (P<0.05) cytotoxic as measured in the neutral red assay (based on EC(50) values-microg TPM/ml media) than the CSC from the four test cigarettes regardless of the regimen used. Collectively, these data demonstrate that the toxicity of CSC from Eclipse is significantly reduced relative to the activity of CSC from the tested vULT cigarettes and the Marlboro Ultra Lights.